The Echinoid Directory

Apical disc and periproct

The apical disc of a regular echinoid
is composed of five ocular plates (labelled I-V) and five genital
plates (labelled 1-5). The ocular
plates are pierced by a microscopic
perforation that houses the tip of the radial water vessel; an
ocular plate is situated at the summit of each ambulacrum. The
genital plates are generally larger and are pierced by a much
larger pore, the gonopore. It is through this opening that the
echinoid releases eggs or sperm into the water column. One of
these plates, G2, is larger and perforated by a mass of small
holes, the madrepores (visible under a microscope). This circle
of plates surrounds a flexible plated membrane termed the periproct.
At the centre of the periproct is the anus. The outer margin
of the periproct may be smooth and oval or subcircular, or (as
in the specimen above) angular. Plating of the apical disc may
be firmly sutured to the corona (and thus commonly preserved
in fossils), or may be imbricate and only loosely bound to the
corona (and thus rarely found in fossils).

In the specimen illustrated above the
circle of genital plates is continuous and the ocular plates
are removed from the periproct, an arrangement that is termed
dicyclic. Where
all five ocular plates lie in contact with the periproct and
separate genital plates, the disc is termed monocyclic.
Some taxa have an intermediate condition, where the anterior
ocular plates are external but one or both of the posterior ocular
plates is insert and interrupts the circlet of genital plates.
This condition is termed hemicyclic. In
some taxa there is a distinctly enlarged periproctal plate, the
suranal plate. This may be firmly integrated
into the circlet of genital plates, or may be present only in
juveniles.

Important variations
in disc plating are illustrated below (ocular plates are green,
genital plates yellow and the periproct is in black).